The present invention relates to an automotive exhaust gas catalyst which has a single-layered, catalytically active coating of high surface area support oxides on an inert carrier structure, wherein the coating contains palladium as the only catalytically active noble metal.
Internal combustion engines emit carbon monoxide CO, unburnt hydrocarbons HC and nitrogen oxides NO.sub.x as the main pollutants in the exhaust gas, a high percentage of these being converted into the harmless components water, carbon dioxide and nitrogen by modern exhaust gas treatment catalysts. Conversion takes place under substantially stoichiometric conditions, that is the oxygen in the exhaust gas is controlled using a so-called lambda sensor in such a way that the oxidation of carbon monoxide and hydrocarbons and the reduction of nitrogen oxides to nitrogen can take place almost quantitatively. The catalysts developed for this purpose are called three-way catalytic converters.
Stoichiometric conditions prevail when the normalized air/fuel-ratio .lambda. is 1. The normalized air/fuel-ratio .lambda. is the air/fuel ratio standardized to stoichiometric conditions. The air/fuel ratio states how many kilograms of air are required for complete combustion of one kilogram of fuel. In the case of conventional gasoline engine fuels, the stoichiometric air/fuel ratio has a value of 14.6. The engine exhaust gas has more or less large, periodic variations in normalized air/fuel-ratio depending on the load and the engine speed. To produce better conversion of oxidizable hazardous components under these conditions, oxygen-storing components such as, for example, cerium oxide are used which bind oxygen when it is present in excess and release it again for oxidative conversion when there is a deficiency of oxygen in the exhaust gas.
Future exhaust gas limits for internal combustion engines require an increasingly stringent reduction in the emissions of hazardous substances in standardized driving cycles. Given the current status of exhaust gas treatment, the hazardous substance emissions which still remain are produced during the cold-start phase. A substantially improved hazardous substance conversion over an entire driving cycle is therefore only possible by reducing cold-start emissions. This can be achieved, for example, by a catalyst with the lowest possible light-off temperature for hazardous substance conversions and/or by locating the catalyst only just downstream of the exhaust gas outlet from the engine in order to reduce the heating-up time required to reach the operating temperature of the catalyst.
If the catalyst is installed near to the engine, it is subjected to exhaust gas temperatures of up to 1100.degree. C. during continuous operation of the engine, and at full speed. Thus catalysts which are temperature-resistant and have long-term stability are required for this type of use.
The present invention deals with catalyst coatings on inert, monolithic support structures, normally honeycomb structures with parallel flow channels for the exhaust gas. The number of flow channels per cross-sectional area is called the cell density. Inert carriers with cell densities between 10 and 250 cm.sup.-2 are used, depending on the requirements of the application. These may be extruded, ceramic carriers made from cordierite, mullite or similar, temperature resistant materials. Alternatively, honeycomb structures made from steel sheeting may be used.
The catalytic coating generally contains several noble metals from the platinum group of the Periodic System of elements as catalytically active components, these being deposited in highly dispersed form on the specific surface area of high surface area support materials. The coating also contains further components such as oxygen-storing materials, promoters and stabilizers. The coating is applied to the internal walls of the flow channels by known coating processes, using an aqueous coating dispersion which contains the various components of the catalyst.
The inert monolithic carriers are also called support carriers in the context of this invention in order to be able to differentiate them more easily from the high surface area support materials for the catalytically active components. High surface area materials are those materials whose specific surface area, or BET surface area (measured in accordance with DIN 66132), is at least 10 m.sup.2 /g. So-called active aluminum oxides satisfy this condition. These are finely divided aluminum oxides which have the crystal structures of the so-called transition phases of aluminum oxide. These include chi, delta, gamma, kappa, theta and eta-aluminum oxide.
The catalyst components may be added to the coating dispersion in a variety of forms:
a) as "finely divided solids"
This is understood to mean powdered materials with particle sizes between 1 and about 50 .mu.m. In the English language literature, the expressions "bulk material" or "particulate material" are used for these.
b) as "colloidal solids"
These have particle sizes of less than 1 .mu.m. The particulate structure of finely divided and colloidal solids is retained even in the final catalyst coating.
c) in the form of soluble "precursor compounds"
Precursor compounds are converted into actual catalyst components only by subsequent calcination and optionally reduction and are then present in a highly dispersed form.
The catalytically active metals from the platinum group or stabilizers such as lanthanum oxide and barium oxide are preferably incorporated into the coating as soluble precursor compounds in the coating dispersion or introduced only later by impregnating the coating. After a subsequent calcination procedure, these materials are present in a highly dispersed form (crystallite sizes in general of less than 5-10 nm) on the specific surface areas of the high surface area, finely divided components of the catalyst. They are also called "highly dispersed materials" in the context of this invention.
The aim of the present invention is to develop a catalyst suitable for use in the area mentioned above which operates exclusively with palladium as the catalytically active noble metal. Palladium is characterized, as compared with platinum, by a lower price, which is important with regard to the economic viability of the catalyst. In addition, it is known that palladium is a very effective catalyst for the oxidation of unburnt hydrocarbons, in particular paraffins and aromatic compounds. It has a superior effect, with reference to the same mass, to that of platinum.
U.S. Pat. No. 4,624,940 describes a three-way catalytic converter in the form of a coating on a monolithic support carrier which contains only palladium as a catalytically active component and which retains its catalytic activity even after aging at temperatures higher than 1000.degree. C. The coating contains at least three different finely divided materials: thermally stable aluminum oxide as support material for a metal from the platinum group, further metal oxides as promoters which do not contain metals from the platinum group and inert, thermally stable fillers. The support material is stabilized with lanthanum, barium and silicon. The lanthanum oxide used for stabilizing purposes may contain up to 10 wt. % of praseodymium oxide. Cerium oxide, zirconium oxide or mixtures thereof are used as promoters. Finely divided cordierite, mullite, magnesium/aluminum titanate and mixtures thereof are used as fillers. Palladium is preferably used as a metal from the platinum group. According to U.S. Pat. No. 4,624,940, care has to be taken to ensure that palladium is not deposited on the cerium oxide-containing promoters because this would impair the effectiveness of both the palladium and the promoter.
U.S. Pat. No. 5,057,483 describes a catalyst composition which consists of two discrete layers on a monolithic carrier structure. The first layer contains a stabilized aluminum oxide as support material for platinum and finely divided cerium oxide. The first layer may also contain finely divided iron oxide and nickel oxide to suppress hydrogen sulphide emissions and also highly dispersed barium oxide and zirconium oxide as thermal stabilizers, these being distributed throughout the entire layer. The second layer contains a coprecipitated cerium/zirconium mixed oxide, onto which rhodium is deposited, and an activated aluminum oxide as support material for platinum. The coprecipitated cerium/zirconium mixed oxide preferably contains 2 to 30 wt. % of cerium oxide.
U.S. Pat. No. 4,294,726 describes a single-layered catalyst composition on an inert carrier structure which has platinum, rhodium and base metals as catalytically active components, these being deposited on an active aluminum oxide. The active aluminum oxide contains cerium oxide, zirconium oxide and iron oxide. The catalyst is obtained by impregnating active aluminum oxide with an aqueous solution of cerium, zirconium and iron salts. After calcining the aluminum oxide treated in this way, it is then impregnated again with an aqueous solution of platinum and rhodium salts.
U.S. Pat. No. 4,965,243 also describes a single-layered, thermally stable, three-way catalytic converter on a monolithic carrier structure which is obtained by coating the carrier structure with a coating dispersion which contains a metal from the platinum group, active aluminum oxide, cerium oxide, a barium compound and a zirconium compound.
WO 95/00235 describes a two-layered catalyst on an inert carrier structure which contains only palladium as a catalytically active component. The first layer contains a first support material and at least one first palladium component and a first oxygen-storing component which is in intimate contact with the palladium component. The second layer contains a second support material and at least one second palladium component. .gamma.-aluminum oxide is used as a first support material and palladium is deposited on this by impregnating with an aqueous palladium nitrate solution. The aluminum oxide obtained in this way is processed with a colloidal dispersion of cerium oxide (particle size about 10 nm), cerium nitrate crystals, lanthanum nitrate crystals, barium acetate crystals, a zirconium acetate solution, a cerium/zirconium mixed oxide powder and a nickel oxide powder to give a coating dispersion for the first layer. For the second layer, a coating dispersion is made up which contains aluminum oxide coated with palladium in the same way as for the first layer as well as lanthanum nitrate crystals, neodymium nitrate crystals, zirconium nitrate crystals and strontium nitrate crystals. After each coating procedure, the carrier structure is calcined at 450.degree. C. in order to convert the precursor compounds of the various coating components into the corresponding oxides.